1. Field of the Invention
The present invention relates to a surface-acoustic-wave filter using a transversal SAW filter used for a high-frequency circuit of a radio communication unit.
2. Related Art of the Invention
A surface-acoustic-wave filter has been frequently used so far as a RF- or IF-stage filter of a transceiving circuit of a communication unit. Particularly recently, a portable telephone according to a mode referred to as CDMA (Code Division Multiple Access) has been positively developed as a next-generation mobile communication system.
The CDMA communication mode allows the so-called RAKE reception for improving the sensitivity by synthesizing delay waves of a multipath.
A CDMA receiving circuit is described below by referring to the accompanying drawings.
FIG. 9 is an illustration showing the outline of a CDMA receiving circuit. In FIG. 9, symbol 702 denotes a demodulating section, 703a and 703b denote antennas, 704a and 704b denote filters, 705a and 705b denote amplifiers, 706a and 706b denote mixers, 707a and 707b denote transversal SAW filters, and 708 denotes a delay unit. In this case, the transversal SAW filters 707a and 707b are assumed as the same device.
Operations of the CDMA receiving circuit having the above configuration according to the prior art are described below. That is, a signal received by the antenna 703a passes through the filter 704a and amplifier 705a and is converted into an IF signal by the mixer 706a. When assuming the signal as an IF signal xe2x80x9ca,xe2x80x9d the IF signal xe2x80x9caxe2x80x9d passes through the transversal SAW filter 707a and is output to the demodulating section 702.
Similarly, a signal received by the antenna 703b passes through the filter 704b and amplifier 705b and is converted into an IF signal by the mixer 706b. When assuming the signal as an IF signal xe2x80x9cb,xe2x80x9d the IF signal xe2x80x9cbxe2x80x9d passes through the transversal SAW filter 707b and delay unit 708 and is output to the demodulating section 702.
The demodulating section 702 performs RAKE reception by synthesizing the IF signal xe2x80x9caxe2x80x9d and the IF signal xe2x80x9cbxe2x80x9d input by being delayed by the delay unit 708.
Operations of the CDMA receiving circuit are described above.
However, the above configuration has problems that a single delay unit is required for RAKE reception and thereby, the circuit size increases. Therefore, the CDMA receiving circuit cannot be applied to a compact portable terminal unit.
The present invention is made to solve the above problems and its object is to provide a surface-acoustic-wave filter capable of reducing the number of parts for signal delay and downsizing a receiving circuit, and a communication unit using the surface-acoustic-wave filter.
One aspect of the present invention is a surface-acoustic-wave filter, comprising:
a piezoelectric substrate; and
a plurality of transversal surface-acoustic-wave (SAW) filters set on the piezoelectric substrate and respectively having at least two inter-digital transducer (IDT) electrodes for transceiving surface acoustic waves,
characterized in that periods for surface acoustic waves to propagate from inputs to outputs of the SAW filters differ in the SAW filters and thereby, timings of outputs from the SAW filters are different from each other.
Another aspect of the present invention is a surface-acoustic-wave filter, comprising:
a piezoelectric substrate;
a plurality of filters set on the piezoelectric substrate and respectively having at least two inter-digital transducer (IDT) electrodes for transceiving surface acoustic waves; and
a plurality of impedance-matching circuits connected to input sides and output sides of the filters,
characterized in that output timings of output-side impedance-matching circuits of the filters are different from each other.
Still another aspect of the present invention is a communication unit using a CDMA mode, comprising:
receiving means provided with;
a plurality of antennas,
a plurality of mixers for converting signals sent from the antennas into IF signals,
a plurality of surface-acoustic-wave (SAW) filters for passing the IF signals, and
a demodulating section for demodulating the IF signals,
wherein said surface-acoustic-wave filter is used for the SAW filters.
According to the above configuration, a surface-acoustic-wave filter of the present invention is characterized by having transversal SAW filters corresponding to a plurality of reception signals, making group delay times of the transversal SAW filters different from each other, and thereby providing a group delay time difference for IF signals output from the SAW filters and makes it possible to downsize a CDMA receiving circuit by including a delay-unit function in the surface-acoustic-wave filter and reducing the number of parts of the CDMA receiving circuit in accordance with the above configuration.